In a cardiac ablation procedure, radiofrequency energy may be delivered to arrhythmia-causing cardiac tissue, thus causing lesions in this tissue that render the tissue electrically inactive.
Magnetic resonance imaging (MRI) is an imaging technique that acquires complex-valued signals, which may be processed, and used, in various ways. For example, the magnitude of such a complex-valued signal may be computed, to produce a “magnitude image” for viewing by a radiologist. Alternatively or additionally, the phase of such a complex-valued signal may be computed, to produce a “phase image.” Such a phase image may be used, for example, for MRI thermography (or “MRI thermometry”), a technique by which tissue temperature is measured. In one variation of this technique, for example, changes in temperature are derived from differences in image phase. Thus, for example, if a reference image of a particular portion of tissue, acquired at a first time, has the phase ϕ0, and a subsequent image of the same portion of tissue, acquired at a second time, has a different phase ϕ1, the change in temperature of the portion of tissue between the first and second times may be derived from the difference ϕ1−ϕ0. MRI thermography may be used to provide a temperature map of the tissue area of interest; hence, MRI thermography may be alternatively referred to as “MRI-based temperature mapping.”
Yuan, Jing, et al., “Towards fast and accurate temperature mapping with proton resonance frequency-based MR thermometry,” Quantitative imaging in medicine and surgery 2.1 (2012): 21-32, reviews the basic principles of proton resonance frequency (PRF) thermometry, and further discusses technical advancements aimed at increasing the imaging speed and/or temperature accuracy of PRF-based thermometry sequences, such as image acceleration, fat suppression, reduced field-of-view imaging, as well as motion tracking and correction.
Volland, N A, et al., Limited FOV MR thermometry using a local cardiac RF coil in atrial fibrillation treatment,” Proc. Intl. Soc. Mag. Reson. Med. 19 (2011): 1764, investigates the development of a local RF heart coil that would allow the acquisition of coil-sensitivity limited field of view (FOV) MR lesion or temperature images in less than 200 ms per image with high sensitivity.
US Patent Application Publication 2015/0099965, whose disclosure is incorporated herein by reference, describes a catheter-mounted, expandable or set in position, coil for magnetic resonance imaging. The coil has a catheter sheath including an elongated tube with a central axis, the catheter sheath having an opening at an end thereof; an expandable coil including a conductive material connected to an expansion mechanism which, when deployed, maintains the expandable receive coil shape; and a cable running through the catheter sheath, the cable being electrically connected to the coil inductive loop.